By now, many of my readers will already know I’m a fan of tube amps and tube rolling to find the best fit for them. I believe most amps show a preference for some tubes over others, and that those differences make tube rolling a worth while exercise in most cases. Knowing my love of tube amps and tube rolling, I was approached by Little Dot USA to review the new Mk3SE hybrid tube amp and do a bit of tube rolling with it as well. The MK3SE is the 3rd generation Mk3 and the first to be fully balanced from start to finish. Fully balanced tube amps are not usually a budget item as it takes more circuitry and thus expense to do it well. The Mk3Se retails for less than $500 US, has nearly 2 Watts of output power (into a 150Ω load), and offers 4 gain settings to make it easier to match with all kinds of headphones. To top it off, the Mk3se uses the Chinese 6N11 tube which is analogous to the US 6DJ8, the European EC88, and the Russian made 6N23P so tube choices are plentiful and range in price from dirt cheap through ludicrously expensive depending on your taste and budget. With the Mk3SE, it is easily possible to spend more on the tubes in it, than you did on the amp itself. For some this will be a great place to start, for others the danger of becoming tube rich and cash poor is very real.
I usually start out these articles with a bit of background on the tube itself and the options that are available. In this case, that is likely enough material to write several hundred pages on by itself. the 6dj8 family of tubes is one of the most sought after by audiophiles, with nearly every tube maker producing some variety of it between 1958 and 1990, several versions still in production today. and countless variations produced along the way. The first caveat is that the 6dj8 family is also in enough demand that more than a few fakes, relabels, scams, and cons are out there waiting for the uninitiated. Let’s cover the background first, and I’ll mention things to watch for along the way.
The 6DJ8/Ecc88 was developed by Amperex as an improved 6bk7 and first introduced in late 1958. They are 9 pin miniature bases with a frame grid design dual triode internal. The intended use for the 6dj8 was in VHF and UHF amplifiers for television, but due to its frame grid construction, it has a higher transconductance than most other small dual triodes and for that reason found its way into a lot of things other than TVs. Phillips/Amperex also introduced the 7dj8/PCC88 at the same time which was a series-string version of the 6dj8 with a slightly higher heater voltage (7V vs 6.3V). Not too many years after the introduction of the 6dj8, an improved version came along dubbed the 6922 in the USA, and the E88cc in Europe. Most makers continued to produce both the original 6dj8 and the improved 6922 in parallel through the 1960s and into the 1970s. The 6922 became so popular for instrument use, that large military contracts were put out for it both in Europe where it was called the CV2492 and CV2493 and in the USA where it was usually marked as JAN 6922. To give readers an idea of how high demand for the 6922 was in its heyday, Phillips/Amperex built a facility in New York state specifically to produce the 6922 for the US military contract which specified the part had to be USA made. Not only did Phillips make the 6922 for military use, Sylvania made large numbers as well so JAN 6922s are very likely to be one of those two makers. RCA contracted with Siemens of Germany and resold Siemens made tubes as RCA. To my knowledge RCA never produced a 6922 in house.
If having an improved 6dj8 and mil-spec version of that wasn’t enough, an even higher spec version was produced dubbed the 7308 or E188cc. The 7308 was essentially a 6922 with matched triodes and extremely narrow tolerance ranges. Basically any 7308 by a given manufacturer should be a nearly exact match to every other 7308 made by that same maker. Most 7308s were made in the USA for military contracts although they were made for industrial application as well in both the USA and in Europe. 7308s were never as plentiful as the other varieties and were more expensive at the time of production. That trend has continued with prices today starting around 100 USD per NOS tube and climbing to levels closer to 500 USD depending on make, year, and condition.
And guess what, there is an improved version of the 7308 called the CCa. The CCa came into being as the result of a request for purchase by the German telephone system for a 6dj8 tube made with the tightest tolerances possible. Siemens and Telefunken both produced CCa tubes with gold pins, 10,000 hour heaters, frame grid winding kept within 1/2 of a percent tolerance across the production run and every possible test run to assure the resulting tubes were as tightly matched both internally and between samples as possible. The CCa is a great example of what tube makers were capable of if money was not an object and all possible care was taken in the manufacturing process. As one can imagine, these were never plentiful or inexpensive and today they are the holy grail of the 6dj8 family. One can easily spend upwards of $500 a tube for new old stock tubes if they can be found.
You will find ebay sellers marking other tubes as acceptable substitutes for the 6dj8, be very careful and don’t be drawn in by much lower prices for a supposedly equivalent tube as many simply are not usable. One commonly seen example is the 7es8/pcc189 which is a variable mu tube never intended for audio use. A variable mu means different frequencies will be amplified different amounts so, as you can guess, this is not acceptable for audio use and should never be substituted into an 6dj8 socket.
With the collapse of the Soviet Union, the Russian equivalent of the 6dj8 became much more readily available in the west. Depending on your source, you’ll find both the 6N1p and the 6N23p listed as possible substitutes for the 6dj8. The 6n23P is a true 6922 clone while the 6N1P is not. The 6n1p has much higher heater current requirements and different maximum anode voltage, mutual conductance, and internal resistance ratings that will require significant circuit changes in order to accommodate. Here again, buyer beware as many sellers will mark the 6N1p as a 6dj8/6922 in order to sell the tons of NOS military surplus tubes on the market.
The Chinese equivalent is the 6N11 which at first glance looks to be a Russian tube designation, but to my knowledge there is no 6N11 in the Russian tube designations.
So that gives us options of 6dj8, ecc88, 6922, e88cc, cv2492, cv2493, e188cc, 7308, CCa, 6n23p, and 6n11 which are all very nearly identical, as well as the 7dj8, and pcc88 that differ in heater voltage but are otherwise the same. Arguably, all of these are the same tube with variance in tolerances between triodes and variances between tubes held to differing levels. A great pair of 6dj8s may sound just as good as a great CCa but the odds of finding that great pair are much higher if starting out with the premium models as tolerances were tighter to begin with. The good news is there are still plenty of reasonably priced good sounding 6dj8 and 6922 labeled tubes to start you on your journey. If you have deeper pockets, the CCa is generally listed as one of the top picks of all small triode tubes for audio use so may be worth the extra spend.
So, since I don’t have an endless budget, I chose a selection of tubes at various price points to find the best options at reasonable prices, and a few unreasonable ones just for fun. (Don’t tell SWMBO what these actually cost).
Below is the list of tubes I pulled out to test. Sad part is this is a small slice of those available so the best tube may still be lurking out there somewhere outside this list. The good news is there are some great ones on the list so all is not lost.
|Budget 6DJ8 Models||Step Up 6922 tubes|
|Tube Maker||Date of Manufacture||Cost Per Pair||Tube Maker||Date of Manufacture||Cost Per Pair|
|RCA 6DJ8||1969||$50||Phillips 6922||1977||$80|
|Sylvania 6DJ8||1969||$40||Gold Lion 6922||current production||$90|
|Amperex 6DJ8||1972||$40||Raytheon 6922||1966||$90|
|GE (Clear Glass) 6DJ8||1976||$40||RCA (Amperex) 6922||1965||$150|
|GE (Black Glass) 6DJ8||1964||$40||Telefunken Ecc88/6922||1967||$300|
|Chinese 6N11||current production||$40||Mullard (West Germany) Ecc88||1982||$70|
|Reflector 6N23P||1982||$40||Tesla 6922 (crossed sword -blue tip)||1986||$200|
|Premium Tubes (7308/CCA)||Alternate tubes (7DJ8)|
|Tube Maker||Date of Manufacture||Cost Per Pair||Tube Maker||Date of Manufacture||Cost Per Pair|
|Reflector 6N23P-EV (swgp)||1974||$150||Telefunken PCC88||1964||$90|
|Amperex 7308 PQ||1964||$350||Phillips PCC88||1972||$90|
|Phillips 7308 SQ||1961||$300||Amperex 7DJ8||1966||$150|
|Mullard 7308 (West Germany)||1966||$160|
|RCA 7308 (Siemens)||1963||$300|
As you can imagine, with 24 tubes to try out, this is a huge undertaking. The fact that I found the Mk3se worthy of this kind of effort speaks volumes, but for a full review, look here.
First lets take a look at the 6DJ8 budget class tubes. All of these can generally be had for less than $50 a pair, and when properly matched, these can be a great value. The 6922 does not inherently sound any better than the 6DJ8, it just had tighter factory QC and smaller tolerances so the likelihood of finding a well matched pair is improved. All of my pairs of 6DJ8s were very tightly matched so keep that in mind when looking for a good option.
|RCA 6DJ8||There is some question in my mind as to whether these tubes are Amperex rebranded as RCA, the internals look suspiciously like an Amperex NY made tube, but I cannot find any supporting documentation that RCA rebranded Amperex NY 6DJ8 tubes. Sound quality is very similar to the Amperex with good mids and top end, if a little bit lighter bass impact than some others in the line up.|
|Sylvania 6DJ8||These were surprising in that they ranked almost even with the 6N23p’s in this testing and were on par with some of the 6922 and premium tubes in many regards. Extension was very good, stage was very good, mids were lush and plentiful. If these had a drawback it was a slightly grainy treble|
|Amperex 6DJ8||This is the tube that started it all so it was only appropriate that I give it a shot. The Amperex had good mids and highs with a touch less bass impact than some in the round up. Stage size was good but not the best of the bunch either. Good all-around, but not great.|
|GE (Clear Glass) 6DJ8||These were a disappointment, these had good low end but mids were less so and treble was grainy and a bit forward. Stage was nothing out of the ordinary so these were probably my least favorite of the budget tubes.|
|GE (Black Glass) 6DJ8||The black glass fools some people into thinking they have a problem or have failed. The black coating was originally done to reduce RF interference. It was dropped in later years as it was found not to significantly do that. These had a good tone, nice extension with good lows, and good stage. A good all around tube and a safe pick for a budget 6DJ8.|
|Chinese 6N11||This is the stock tube shipped with the Little Dot Mk3se and while not the best of the lot, it wasn’t the worst either. The 6n11 had good low end, good mids, a bit of grain in the treble, and reasonable stage size. Overall, the 6N11 was more competent than the 12ua7 or 5751 versions I have previously tried from the same factory.|
|Reflector 6N23P||These were all built to military spec as they were not offered for civilian sale and as such these are closer to the 6922 than the 6DJ8 in many respects. There were my top pick of the budget tubes with good extension on both ends, lush mids, nice thump at the low end, and great stage. The good news is with the military ordering millions of these, there is no shortage and prices should remain low for some time. If I were purchasing the Mk3se on a tight budget, I’d buy 8 of these and feel well supplied for a very long time.|
|Voskhod 6N23P||This was the other Russian maker of 6N23P tubes for military service and internals are very similar to the reflector. Performance was also very similar. The Voshkod did need a good bit of time to lose some grain in the treble, but once burned in, it was as good as any in this price class and better than most. If these can be found cheaper than the Reflector made tubes that are getting a reputation, grab them.|
So among the budget class tubes there are some good options and honestly I think many will be well served with the tubes that shipped with the Mk3se, but it can certainly be improved with either the Sylvania, GE Grey glass, or Russian 6N23p tubes if desired. The next step up, is to look at the 6922 options. Here we expect tighter QC and a bit better sonics as a result. So the bar is raised a bit, but so is the price tag with these averaging closer to $100 USD per pair rather than $50.
|Phillips 6922||Very clean, good extension at both ends, very little added warmth so for those looking for something with less warmth, this is a good option. Very crisp and transparent but lacking a bit of impact compared to some others.|
|Genelex Gold Lion 6922||For the hype surrounding this tube and the fact that it is made at the same plant that made the 6n23p reflectors that are so sought after, I expected good things. Sadly, this tube did not live up to its pedigree. Low end extension is good but mid-bass is slightly loose and top end is a bit grainy as well both of which are not acceptable in a premium tube. This may just be a bad match for the Mk3se, and work well other places, but I’d save my money and keep the stock tubes over these in this application.|
|Raytheon 6922||If the Genelex was a surprise let down, these were the opposite. Great lows, warm mids, and good top end extension all at the most reasonable price tag of the 6922s tested. Stage is good sized and quite well rounded and imaging is very good as well. As a budget option, these are probably the best pick. They are harder to source than some others, but worth the hunt if you can get them for less than the Amperex as they are very close in sound quality to the Amperex.|
|Amperex 6922||The Amperex 6922 is a good all around listen. Some weight at the low end brings a bit of warmth, mids are thick (read lush not over-done) and highs have good extension with little or no grain to them. Stage is also good with maybe a hint less size than the Raytheon, but still larger than the Phillips or Genelex.|
|Telefunken Ecc88/6922||These were the most expensive of the 6922s and have a reputation for being one of the top options for 6922 based pre-amps so they get snatched up quickly when they do come up for sale. The reason for this is they have good extension at both ends, very clean sound, and effortless detail. They impart very little warmth so if you are looking for that, they aren’t the choice, but for a clean, well detailed sound if slightly thinner than something like the Raytheon or Amperex, these are the operative choice.|
|Mullard (West Germany) 6922||What is in a name? In this case very little. We associate Mullard with British made tubes with extremely lush mids and more warmth than most. In this case the 6922 is actually made by Siemens in West Germany and sounds like a German tube. It is a bit dry, well extended and detailed, but does not add warmth and the mids are if anything a bit thin. These are a Mullard in name only and the sound explains why these don’t command the premium some Mullard tubes (12At7) do.|
|Tesla 6922 (Crossed Sword-Blue tip)||Winner! these were among my favorites and the best performing tube in cost / performance of the test. These had great lows extension and power, lush mids with a touch of tube warmth but not syrupy, and soaring highs with no grain to be found. These could easily be classified as a 7308 or CCa based on SQ and can be had at a much more pedestrian price. My best budget option of the round up. This pair is from plant 37, has blue tips indicating top 1% of production and were made in early 1986. These have been made for a lot of years at two different factories (32 and 37) in my experience the 37 is held to tighter tolerances and probably the better bet.|
Of the 6922 tubes I am most impressed with the Raytheon and the Tesla. The Raytheon fits in the budget category and gives good service all the way around while the Tesla fits in the premium class and honestly was as good or better than most of the 7308s I tested at a considerable discount. If it weren’t for the Russian 6n23p-EV in the next category, the Tesla would take the top value spot for certain. With the Russian tube being arguably as good as the CCa, the Tesla has to settle for a fairly close 2nd place finish.
|Amperex 7308 PQ||My pair of Amperex 7308s are orange label which are slightly less desirable than the US Made white label tubes, but still command ridiculous prices in today’s market. These have good tight bass with plenty of slam, some warmth to the mids but not enough to be syrupy, and highs have great sparkle and extension. If you have to define the PQ sound in a word, it would be clean. Nothing loose, nothing too forward or to recessed, just well done. It is very hard to fault this tube on performance, where it loses out is on price. It was not markedly better than the Tesla or 6N23pEV (1974) and was easily double the price. (and remember these are the less desirable of the Amperex models so are cheaper than the white label version.|
|Phillips 7308 SQ||Guess what, same tube as discussed above, yep the Phillips SQ came out of the same plants as the Amperex PQ. I included them here as my two pairs are separated by nearly 4 years in production so more an experiment to see if they evolved over the production years. Overall, very similar to the Amperex 7308PQ above and also equally priced so great sound, but price/performance not as good as some others.|
|Mullard 7308 (West Germany)||The Mullard 7308 was made by Siemens in West Germany for Mullard and is not a british made Mullard so sound is not the thick, warm, fluid, sound we normally expect from that name. Instead, these have great lows, clean but slightly less forward mids than one would expect from this name, good top end extension and good stage. Imaging is solid,but not class leading. Remember that Siemens made tubes for Mullard, Siemens, Phillips, and RCA among others so it is possible to find the same tube with several brands and multiple price points.|
|Reflector 6N23p-EV (swgp)||These are the silver shield getter on the wire model that are some of the hottest tubes going right now, so I had to include them to see if they hype is warranted. In short, it is. These were among the top performers with stage larger than all but maybe the Tesla and it was too close to call. Bass is solid, mids are well detailed and clean, top end extension is great with no grain to be found. All in all a stellar tube and one that will rival CCa at a fraction of the cost. The 2nd best value tube in the round up on a cost/performance basis for a true 6922 behind the Tesla and the difference is because the Tesla is less expensive not better sounding.|
|RCA 7308 (Siemens)||The RCA 7308 was made for RCA by Siemens in West Germany similar to the Mullard listed above. These are generally less expensive than their Siemens branded counterpart but more than the similar Mullard branded tube although all three came from the same plant. I found these to be a very good listen as well with great lows, clean but slightly less forward mids than the 6N23p-EV mentioned above, good top end extension and good stage. With the similarity to the Siemen’s made Mullard tube, I’d search out the less expensive tube first.|
|Siemens CCa||The Holy grail of the 6922 family without doubt is the CCa. Ridiculously expensive and often faked so be careful when purchasing, these are the standard by which all 6922/6DJ8 tubes are judged and with good reason. These are a fantastic sounding tube. They have everything going for them from big bass when called upon, lush mids with enough warmth to let you know this is tube audio but without getting any thickening or syrup. Highs seem to go on indefinitely and are as clean as to be found. If you have the funds, these are worth it, but honestly, with the Tesla and Reflector coming in at 97% of the CCa (and maybe 98% at times) its hard to justify the cost as anything other than showing off.|
Alternate Tubes: It is often warned that the PCC88 may not work in all amps designed for the 6DJ8 as they have a higher heater voltage requirement. That being said, in my experience these tubes will operate with 6V provided to the heater and will often outlive their 6922 counterparts as a result of being mildly under-heated. Other than the 7 volt heater, these are exactly the same construction as the same makers 6922 in most cases. Your mileage may vary, but these give many the option to purchase a tube equal to the 6922 at a lot lower cost. Telefunken 6922 tubes generally run $100-$150 per tube while the Pcc88 version can be had for less than $100 per pair. The Pcc88 was not a premium tube so isn’t a match for the 7308, but offers a budget minded consumer a way to try some of the high end 6922 models that might be otherwise out of financial reach.
|Telefunken PCC88||I bought my stock of these in Germany as they are much easier to find there than in the US. They are however, worth searching out as they share the classic Telefunken sound with their 6922 siblings. These had great detail and air, good thump, a large stage, and a very low noise floor. While they are not billed as a premium tube, they sound an awful lot like the 7308s in this round up for about 1/3rd the price. Recommended.|
|Phillips PCC88||Unlike the previous section, this tube is not the exact same tube as the Amperex below. In this case the Phillips is a USA made tube while the Amperex is a Holland made tube. Both were made by the same company, but on different continents at different dates so the question is do they share a signature or does each have its own unique sound? I found these very similar to the Amperex Holland, but with a touch less warmth in the mids and a bit more low end extension. These seem to have taken the thickness of the mids and slid it down into the mid bass. Top end has good air and helps open the stage a bit further than the Holland made version. The good news is these are 1/2 the price of the other (yeah makes no sense) so grab these while they are cheap if the signature matches your desires.|
|Amperex 7DJ8||While these are Valvo labeled, all markings show they were made at Amperex Holland and rebranded by Valvo so they are certainly identical in all respects to those labeled as Amperex tubes of the same era. One advantage to the Valvo tube is it has the A-frame getter which is a little more stable and less likely to be microphonic. These have the same signature as the Amperex 6922 listed above with good low end weight, a bit of warmth and thickness to the mids, and good extension and air up top. Stage is good but not quite class leading. Unlike some others, these seem to have been discovered as the price on the Amperex 7DJ8 and 6922 run about the same so grab the one with the better price tag.|
So there you have it, 24 tubes rolled into the Little Dot Mk3Se and listened to for countless hours using the Hifiman He6 to pick out as many tiny details as possible. I’ll admit some days this was fun, others it gave me a headache. There are several conclusions that can be drawn from all that listening, probably not the least of which is I should spend less time on audio and more with family. That aside, in general, 6922 tubes did sound improved over the 6DJ8 base models, 7308s used to be worth the extra cost but with the fall of the iron curtain, tubes like the Reflector EV and Tesla have become more readily available in the west and cast the 7308s in a less enviable light as over-priced. The same goes for the CCA branded tubes as while they are spectacular, they should be for the cost, and even they have trouble separating themselves from the best of the 6922 and 7308 models. On a price performance basis the availability of the Eastern block tubes has set the market a bit on its ear as the base model 6n23p gives most of the western 6922s a run for their money at $40-50 a pair and the best of the 6N23p (Reflector Silver Shield 1974 EVs) rival CCa (at least in this application) for about 1/4 of the asking price. I’m sure the best of the Reflectors will only gain in value much as the 7308s and CCa have so it may well be wise to grab a few pairs while they are reasonably priced (I’ve never seen a tube get cheaper over time). Same goes for the Tesla blue tips, at $200 they were very close to the CCA sonically and at least as good as any of the 7308s in the round up. If these were cheaper, they might actually displace the Russian tube for the top spot but as it is, the cost keeps them in position 2.
Best Value tube – Sylvania 6DJ8 / Raytheon 6922 / PCC88 Telefunken
Best mid priced tubes – Reflector 1974 EV, Tesla 6922 Crossed swords (blue tips)
Best money is no object tubes – CCa, Reflector 1974 EV, Tesla Crossed Swords (blue tips)
For more information on the Little Dot MK3SE, see my full review here.
For the record, SWMBO just pointed out “So you bought and tested $3400 worth of tubes in a $500 amp?” While I had a good number of these in my collection before the Mk3se arrived, this is a good example of what can happen if you get into tube rolling and don’t pay careful attention to what you are buying. Remember a good tube can last 10,000 hours so a single set of tubes would last roughly 3.5 years if used 8 hours a day. Most of us don’t get 8 hours a day listening time so they actually can last considerably longer. If a good pair of tubes can last 10 years for many users, having a single set of spares on hand may well be all you need. My hope is that this article will help you find your best tube quickly and save you a few dollars in the process. Its too late for me, SWMBO has decided my purpose in life is to serve as a warning to others. Happy rolling!